A non-aqueous electrolyte secondary battery, particularly a lithium secondary battery has high energy density, and is therefore widely used as a battery for a personal computer, a mobile phone, a portable information terminal and the like, and recently has been being developed as an on-board battery.
As a separator in a non-aqueous electrolyte secondary battery such as a lithium secondary battery, a porous film having a polyolefin as a principal component, or a laminated porous film obtained by laminating other functional layers with the porous film as a base material is used.
Such a porous film has a structure in which coupled pores are provided in the film, so that a liquid containing ions can permeate from one surface to the other surface through the coupled pores. Therefore, the porous film is suitable as a separator member for a battery in which ions are exchanged between positive and negative electrodes.
Preferably, the porous film has a high porosity from the viewpoint of improvement of the ion permeability. On the other hand, when the pore diameter of pores in the porous film is excessively large, dendrite generated in a negative electrode easily arrives at a positive electrode to cause a short-circuit when the porous film is used as a separator of a battery. Therefore, the pore diameter of the porous film is preferably as small as possible.
Examples of the method for controlling a porous structure of a porous film include a method in which a resin sheet as a raw material is uniaxially or biaxially stretched.
For example, Patent Document 1 discloses a method for producing a porous film in which a porous structure is controlled by stretching a sheet with the strain rate changed in the initial stage to the final stage in stretching while making the stretching temperature constant. Patent Document 2 discloses a method in which the temperature is changed between the upstream stage and the downstream stage in stretching to perform heat-setting. Patent Document 3 discloses a method for producing a thermoplastic resin film in which the temperature in a stretching step is made lower than the temperature in a preheating step in a simultaneous biaxial tenter stretching method.
Usually, however, when a porous film is produced under conditions that increase the porosity, the pore diameter of pores tends to increase, so that it is difficult to balance the porosity and the pore diameter of the film at a practical level.